Radiant energy is used in a variety of manufacturing processes to treat surfaces, films, and coatings applied to a wide range of materials. Specific processes include, but are not limited to, curing (i.e., fixing, polymerization), oxidation, purification, and disinfection. Processes employing radiant energy to polymerize or effect a desired chemical change are rapid and often less expensive compared to a thermal treatment. The radiation can also be localized to control surface processes and allow preferential curing only where the radiation is applied. Curing can also be localized within the coating or thin film to interfacial regions or in the bulk of the coating or thin film. Control of the curing process is achieved through selection of the radiation source type, physical properties (for example, spectral characteristics), spatial and temporal variation of the radiation, and curing chemistry (for example, coating composition).
A variety of radiation sources are used for curing, fixing, polymerization, oxidation, purification, or disinfections applications. Examples of such sources include, but are not limited to, photon, electron, or ion beam sources. Typical photon sources include, but are not limited to, arc lamps, incandescent lamps, electrodeless lamps and a variety of electronic and solid-state sources (i.e., lasers). Conventional arc type UV lamp systems and microwave-driven UV lamp systems use tubular bulb envelopes made of fused quartz glass or fused silica.
FIG. 1 is a perspective view of a microwave-powered UV curing lamp assembly showing an irradiator and a light shield assembly in the prior art. FIG. 2 is a partial cross-sectional view of the lamp assembly of FIG. 1 showing a half-elliptical primary reflector and a light source of circular cross-section. FIG. 3 is a partial cross-sectional internal view of the light shield assembly of FIG. 1 showing a half-elliptical primary reflector and a light source of circular cross-section mated to a secondary reflector and end reflectors.
Referring now to FIGS. 1-3, the apparatus 10 includes an irradiator 12 and a light shield assembly 14. The irradiator 12 includes a primary reflector 16 having a generally smooth half-elliptical shape with openings 18 for receiving microwave radiation to excite a light source 20 (to be discussed herein below), and a plurality of openings 22 for receiving air flow to cool the light source 20. The light source 20 includes a lamp (e.g., a modular lamp, such as a microwave-powered lamp having a microwave-powered bulb (e.g., tubular bulb with a generally circular cross-section) with no electrodes or glass-to-metal seals). The light source 20 is placed at the internal focus of the half-ellipse formed by the primary reflector 16. The light source 20 and the primary reflector 16 extend linearly along an axis in a direction moving out of the page (not shown). A pair of end reflectors 24 (one shown) terminate opposing sides of the primary reflector 16 to form a substantially half-elliptical reflective cylinder. The light shield assembly 14 of FIG. 1-3 includes a secondary reflector 25 having a substantially smooth elliptical shape. A second pair of end reflectors 26 (one shown) terminates opposing sides of the secondary reflector 25 to form a substantially half-elliptical reflective cylinder.
A work piece tube 30 of circular cross-section is received in circular openings 28 in the end reflectors 26. The center of the openings 28 and the axis of the work piece tube 30 are typically located at the external focus of the half-ellipse formed by the primary reflector 16 (i.e., the foci of the half-ellipse formed by the secondary reflector 25). The work piece tube 28 and the secondary reflector 25 extend linearly along an axis in a direction moving out of the page (not shown).
In operation, gas in the light source 20 is excited to a plasma state by a source of radio frequency (RF) radiation, such as a magnetron (not shown) located in the irradiator 12. The atoms of the excited gas in the light source 20 return to a lower energy state, thereby emitting ultraviolet light (UV). Ultraviolet light rays 38 radiate from the light source 20 in all directions, striking the inner surfaces of the primary reflector 16, the secondary reflector 25, and the end reflectors 24, 26. Most of the ultraviolet light rays 38 are reflected toward the central axis of the work piece tube 30. The light source 20 and reflector design are optimized to produce the maximum peak light intensity (lamp irradiance) at the surface of a work product (also propagating linearly out of the page) placed inside the work piece tube 30.
Microwave-powered, UV-emitting electrodeless lamps used for the light source have several disadvantages. Microwave-powered, UV-emitting electrodeless lamps are bulky, noisy, and require a large manufacturing and distribution infrastructure due to many consumable parts, since the service lifetime of an electrodeless lamp is relatively short. With present day optics, the focused beam width of an electrodeless lamp is at best about 1 centimeter (comparable to the bulb size), which results in a large amount of wasted light energy that does not strike the work product. In addition, a large amount of energy is also wasted as heat in plasma-based lamp systems (electroded or electrodeless lamps). Since lamps often contain a small amount of mercury, they pose an environmental disposal hazard. In current operation, hazardous operating conditions for personnel when assembling and handling such lamps were alleviated with personal protective equipment and lengthy operating procedures.
Accordingly, what would be desirable, but has not yet been provided, is an environmentally friendly, efficient solid state light source that provides high peak UV curing irradiance.